Published European patent document EP 1 173 674 describes a drive train of a motor vehicle. The drive train includes an internal combustion engine, which is associated with an electric machine which generates a torque when the internal combustion engine is started. A clutch, via which a torque generated by the internal combustion engine is transmitted to at least one drive wheel of the vehicle, is provided between the internal combustion engine and a transmission. Devices are provided which, when the internal combustion engine is started, operate the clutch in such a way that a first portion of the torque generated by the electric machine when the internal combustion engine is started is transmitted to the at least one drive wheel of the vehicle and a second portion of the torque generated by the electric machine, which is sufficient for starting the internal combustion engine, is transmitted to the internal combustion engine. The clutch is operated with the aid of a control device, taking into account temperature and/or rotational speed-dependent characteristics maps for the drive torque of the internal combustion engine and/or for the start torque and/or for the clutch torque which is predominantly a function of the clutch engagement path.
In other vehicle drives in use such as hybrid drives having at least one internal combustion engine and at least one additional electric drive, the motor vehicle may be moved only with the aid of the at least one electric drive, depending on the design of the drive train. In this case, the at least one electric drive delivers the entire drive power, whereas the internal combustion engine remains switched off. In the “electric drive” operating mode the internal combustion engine at rest may be started by the at least one electric drive. This may take place, for example, with the aid of a pulse start, the angular momentum of the at least one rotating electric drive being used to set the internal combustion engine at rest into rotation. In order to perform the pulse start, first the rotational speed of the at least one electric drive is increased. In order not to cause the rotational speed of the at least one electric drive to increase suddenly, a clutch situated between the at least one electric drive and a vehicle transmission is usually operated by torque regulation and, during this operating phase of the motor vehicle equipped with a hybrid drive, transmits the driver input torque. After the rotational speed of the at least one electric drive has been increased, a clutch situated between the at least one electric drive and the internal combustion engine is engaged. The disadvantage of this procedure is that the clutch between the vehicle transmission and the at least one electric drive operated in a slip state cannot be operated for a longer period in the slip state which, on the one hand, is necessary for maintaining a continuous output speed; however, on the other hand, it would result in a mechanical or thermal overload on the clutch. This precludes operating the clutch between the at least one electric drive and the transmission of the vehicle equipped with a hybrid drive for a longer time period. To perform the above-mentioned pulse start, the rotational speed at the at least one electric drive of the hybrid drive must first be increased so that the overall start process is extended, since the at least one electric drive must be accelerated first. Only thereafter may a drive connection to the internal combustion engine be established and, at the same time, complex torque regulation of the clutch of the at least one electric drive to the vehicle transmission may take place in order not to generate any sudden torque changes at the output.
In certain types of vehicle drives, such as vehicles having compression-ignition engines, no high vacuum is formed in the intake manifold during operation. This is true for the above-mentioned compression-ignition engines, but also for internal combustion engines that are equipped with direct gasoline injection. In these vehicle types, a vacuum-driven power-assisted braking may be adequately supplied with the aid of a vacuum pump additionally flanged to the internal combustion engine. This vacuum pump directly coupled to the internal combustion engine is continuously driven at a rotational speed that is a function of the engine speed, which, however, as a rule, represents a drive ill-adapted to operating requirements.